The previous manufacturing methods and the use of materials with a radial gradient of a physical material parameter can be illustrated using the example of the manufacture of gradient-index-lenses. In the case of these lenses, glasses are used that have a radial gradient of the index of refraction. The index of refraction n (r) as a function of the radius r from the midpoint of the discoid lens preferably has a quadratic curve ##EQU1## where n.sub.0 and n.sub.1 are, respectively, the indices of refraction in the center at r=0 and at the edge at r=R.
Such gradient-index-lenses can be manufactured from glass by means of ion exchange. In this process, a cylindrical glass rod is placed in a salt bath, for example, which contains ions that are exchanged for other ions in the glass by means of in- or out-diffusion at high temperatures. Since with the ion exchange the composition of the glass changes in the diffusion zones, the index of refraction also changes with the concentration of ions exchanged at any given time. If the rods are cut into discs and these discs are polished, gradient-index-lenses are obtained. This method has several disadvantages that have prevented a wider use up to now:
(a) The desired n(r) curves can be adjusted to a sufficient precision only with difficulty. PA1 (b) The diffusion processes run very slowly over great distances, at higher temperatures as well, with the result that long diffusion times are required and gradient-index-lenses are manufactured with only a relatively small diameter. PA1 (c) The variation .DELTA.n of the index of refraction due to the ion exchange is small (at a maximum, .DELTA.n.apprxeq.0.15), since only ions with comparatively slight concentration and a small contribution to the electronic polarizability of the glasses can be exchanged. PA1 (d) Only in the case of a small number of glasses can a sufficient ion exchange be carried out, with the result that only a limited selection of possible dispersion curves n (r, .lambda.) can be produced. Because of this, the correction possibilities are restricted.
Furthermore, rods having a radial gradient of the index of refraction can be produced by depositing material having variable composition in layers. For this purpose, CVD-methods (chemical vapor deposition), evaporation and similar methods are employed. In order to attain the necessary homogeneity, only a thin layer of material can be deposited at a time in this method, and the composition must change from layer to layer. For this reason, long manufacturing times are required, with the result that these methods are not economically viable in many cases.